This project is a continuation of a long-range program of research on the behavioral and neurochemical processes that may underlie the compulsive use of drugs. The basic assumptions are: 1) that agents such as morphine and cocaine are abused because the drugs themselves are powerful reinforcers, and 2) that reinforcement processes are regulated by specialized systems of catecholamine and endorphin neurons, and 3) that the reinforcing properties of cocaine-like and morphine-like drugs derive from the pharmacological similarities between these agents and the catecholamine and endorphin reinforcement systems, respectively. The research plan is designed to investigate the validity of these major hypotheses. Three different methods will be used to measure the reinforcing action of endorphin and catecholamine treatments: drug self-administration, brain self-stimulation, and single-trial avoidance learning. Specific objectives include assessment of the relative reinforcing potencies of met- and leu- enkephalin and beta-endorphin and determination of their principal action sites in the brain. If the findings support the working hypothesis that catecholamine and endorphin systems both have an important role in reinforcement, then we will need to know whether these systems act separately or jointly, whether their roles are unique or overlapping, and precisely which endorphin and catecholamine systems are involved. The results are expected to provide a framework for understanding the addictive properties of opiate and stimulant drugs.